power mosfet - empaempa.com/dokumanlar/st2016/training-llc.pdf · power mosfet antonino gaito...

LLC resonant

Power MOSFET

Antonino Gaito

Market & Application Development Section Manager

[email protected]

ASIA-PACIFIC REGION

Outline

LLC RESONANT Half Bridge

1. SRC - Series LC tank – overview

2. PRC - Parallel LC tank – overview

3. SRC + PRC = SPRC Series Parallel LC tank – overview

4. LLC resonant

5. MOSFET body diode recovery (possible root cause)

6. Our product target

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1. SRC - Series LC tank - overview


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LC series functioning

Let’s talk about one resonant circuit

type: the SERIES RESONANT TANK.

The maximum gain transfer is

obtained when input signal

frequency (fi) is equal to the

resonant frequency (fr), that is when

the LC impedance is null, and so

into the voltage divider the all Va

voltage is reported on the R.


L C

R Va

Vo

Va

Vo

fr

fi

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L C

R Va

Vo

Va

Vo

fs

fi

By increasing the R value (null

ideally) the factor Q decrase.

So at a fixed fs the shifting between

Va and Vo increases.

R

LQ o

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Vo

This is a real LC series circuit. The

input signal Va is fixed by MOSFETs

switching sequence, so frequency

switching “fs” is equal to “fi”.

Vbus

Q1

Q2

GND

L L out

C out Va Load

C

Va

Vo

fr

fi

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Vo

The R value now is due to the

reflected load value (Req).

Vbus

Q1

Q2

GND

L L out

C out Va Load

C

= Req

Va

Vo

fr

fi

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Vbus

Q1

Q2

GND

L L out

C out Va Load

C



Req

Vo

Va

Vo

=

In a real LC series half bridge circuit

lower load, higher R and so lower

Q.

fr

fi

NO LOAD

FULL LOAD

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Vbus

Q1

Q2

GND

L L out

C out Va Load

C



Vo

Va

Vo

The fr is a boundary between two

operation mode:

Zero volage switching (ZVS) and

Zero current switching (ZCS)

ZVS

fr

fi

ZVS

(fs>fr)

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Vbus

Q1

Q2

GND

L L out

C out Va Load

C



Vo


operation mode:



ZVS

inductive zone

Ir

VaZ

Z

Ir

Ir

Va

(fs>fr)

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Vbus

Q1

Q2

GND

L L out

C out Va Load

C



Vo

Va

Vo


operation mode:



fr

fi

ZCS

(fs

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Vbus

Q1

Q2

GND

L L out

C out Va Load

C



Vo


operation mode:



Capacitive zone

Ir

VaZ

Z

Ir

ZCS

(fs

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Vbus

Q1

Q2

GND

L L out

C out Va Load

C


Vo

Va

Vo

It can be seen from the operating

region that at light load, the

switching frequency need to

increase to very high to keep output

voltage regulated. This is a big

problem for SRC.

fr

fi

1

DRAWBACKS

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Vbus

Q1

Q2

GND

L L out

C out Va Load

C

DRAWBACKS


Vo

Va

Vo

It can be seen from the operating

region that at light load, the

switching frequency need to

increase to very high to keep output

voltage regulated. This is a big

problem for SRC.

fr

fi

High turn off current at high input

voltage condition.

1

2

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2. PRC - Parallel LC tank - overview


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LC parallel functioning

Let’s talk about one resonant circuit

type: the PARALLEL RESONANT

TANK.

The maximum gain transfer is

obtained when input signal

frequency (fi) is equal to the

resonant frequency (fr).


L

C R Va

Vo

Va

Vo

fr

fi

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LLC RESONANT Half Bridge V

a

Vo

fs

fi

By decreasing the R value (infinite

ideally) the factor Q decrase.

L

RQ

o

L

C R Va

Vo


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Vo

In a real LC parallel half bridge

circuit lower load, higher R and so

higher Q.

Vbus

Q1

Q2

GND

L L out

C out Va Load

C

Va

Vo

fr

fi

Req =

FULL LOAD

NO LOAD


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Vbus

Q1

Q2

GND

L L out

C out Va Load


Vo

Va

Vo


operation mode:



fr

fi

ZCS

(fsfr)

C


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Vbus

Q1

Q2

GND

L L out

C out Va Load


Vo

Va

Vo

fr

fi

C

DRAWBACKS

Compare with SRC, the operating

region is much smaller. At light

load, the frequency doesn't need to

change too much to keep output

voltage regulated. So light load

regulation problem doesn't exist in

PRC.

1

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Vbus

Q1

Q2

GND

L L out

C out Va Load


Vo

Va

Vo

fr

fi

C

DRAWBACKS

Compare with SRC, the operating

region is much smaller. At light

load, the frequency doesn't need to

change too much to keep output

voltage regulated. So light load

regulation problem doesn't exist in

PRC.

High turn off current at high input

voltage condition (by simulation

these values are higher than the

SRC ones).

1

2

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3. SRC + PRC = SPRC

Series Parallel LC tank - overview


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L

C R Va

L C

R Va

SRC PRC

+ =

We should overcome the SRC and PRC drawbacks.

We want to:

1. Reduce the high turn off current at high input voltage (from the SRC)

2. Obtain no high frequency changes to regulate at no load (from the PRC)

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We should overcome the SRC and PRC drawbacks.

We want to:

1. Reduce the high turn off current at high input voltage (from the SRC)

2. Obtain no high frequency changes to regulate at no load (from the PRC)

C2

L C1

R Va

SPRC

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The SPRC has two resonant frequencies. One low resonant frequency determined

by series resonant tank L and C1. One high resonant frequency determined by L

and equivalent capacitance of C1 and C2 in series; usually working at its highest

one is more efficient.

C2

L C1

R Va

SPRC

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The SPRC combines the two good characteristics of SRC and PRC, but unfortunately

the operation between the two resonant frequencies is in ZCS working.

C2

L C1

R Va

SPRC

ZCS

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4. LLC resonant


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In order to create a ZVS functioning between these two frequencies we substitute one

capacitor with a inductance, by obtaining the LLC resonant circuit.

C2

L C1

R Va

SPRC (LCC)

L1 C

R Va

LLC

L2

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The benefit of LLC resonant converter is narrow switching frequency range with light

load and ZVS capability with even no load.

L1 C

R Va

LLC

L2

Vbus

Q1

Q2

GND

Lr L out

C out Va Load

Vo

Cr

Lm

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There are two resonant frequencies:

rr

r CLf

2

11

LLC functioning Vbus

Q1

Q2

GND

Lr L out

C out Va Load

Vo

Cr

Lm

For the LLC works like a

SRC tank. This functioning is

present at high load condition, that

is when the Lm is paralleled with a

low impedance.

1rsff

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There are two resonant frequencies:


Q1

Q2

GND

Lr L out

C out Va Load

Vo

Cr

Lm

rmr

r CLLf

)(2

12

For the LLC works like a

PRC tank. This functioning is

present at low load condition. In

this region we do not work because

of the ZCS functioning.

2rsff

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In the frequency range


Q1

Q2

GND

Lr L out

C out Va Load

Vo

Cr

Lm the two functioning are mixed.

12 rsrfff

2rf 1rf

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LLC functioning

the two functioning are mixed.

12 rsrfff

Vbus

Q1

Q2

GND

Lr L out

C out Va Load

Vo

Cr

Lm

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LLC functioning

the two functioning are mixed.

12 rsrfff

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We plot three operating regions.

LLC functioning

ZVS Region 1

ZVS Region 2

ZCS Region 3

Region 3

Region 1

Region 2

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LLC Benefits


1. No high switching frequency at low load

2. ZVS functioning guaranteed by Lm (even with zero load)

3. Turn off current not related to load but only to Lm

4. Secondary diodes do not recover

HIGH EFFICIENCY

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5. MOSFET body diode recovery

(possible root cause)


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During a low to high load transition the driving circuit should be able to oblige

MOSFET to move in ZVS and positive turning off current zone. If this is not

guaranteed MOSFET could work in a dangerous zone.

ZVS

ZCS ZVS

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LOW LOAD HIGH LOAD

After the load variation (from low to high) the switching

frequency should follow the new resonant frequency. If this

does not happen (like in the picture), the system state passes

across the Region 3 and ZVS on and positive turn off drain

current are missed. So when MOSFET is turned off current is

flowing also through its body diode; because of the antagonist

MOSFET turning on a body diode recovery can occurs.


ZVS

ZCS

ZVS

ideal

real

At low load steady state condition the system is

working near the lower frequency resonant.

ZVS on and positive turn off drain current is

guaranteed.

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6. Our product target


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1. Low Coss_eq to help the ZVS transition

2. Low Vsd to reduce the losses during the body diode conduction

3. Improve the dv/dt capability


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power mosfet - empaempa.com/dokumanlar/st2016/training-llc.pdf · power mosfet antonino gaito...

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